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Stages of aerobic respiration

Structure of a mitochondrion - there is an aouter double membrane, with folds called cristae which is where the cytochrome system is located. Within the mitochondrion is the matrix, which is the location of the Kreb's Cycle.

The structure of a mitochondrion

The mitochondria are the site of aerobic respiration.

This has three stages:

  • glycolysis
  • Krebs cycle
  • cytochrome system


Glucose (6C) converted to 2 molecules of pyruvic acid (3C). In the process there is a net gain of 2 x ATP, and the hydrogen acceptor (NAD) is reduced to NADH2

This stage takes place in the cytoplasm [cytoplasm: The living substance inside a cell (not including the nucleus).; it does not require oxygen:

  • Glycolysis is the breakdown of 6-carbon glucose into two 3-carbon pyruvic acid units.
  • The hydrogens removed join with the hydrogen carrier NAD to form NADH2.
  • Although some energy is needed to start glycolysis there is an overall net gain of 2 ATP.
  • The pyruvic acid (3C) enters the matrix of the mitochondrion where it is oxidized (i.e. 2H removed) and a carbon dioxide is lost. Thus forming a two carbon molecule called acetyl-CoA (2C).
  • The hydrogens which have been removed join with NAD to form NADH2.

Krebs cycle: citric acid cycle; tricarboxylic acid cycle

Pyruvic acid is converted to acetyl CoA. CO2 is released and 2H is lost. This is converted to citric acid, which releases CO2 and looses 2H to form 5C compound, which releases CO2 and loses 2H to from 4C compound. This is called the Krebs Cycle. NAD accepts 2H to become NADH2.

This stage takes place in the matrix of the mitochondrion and is the aerobic phase and requires oxygen:

  • It begins when the 2-carbon acetyl CoA joins with a 4-carbon compound to form a 6- carbon compound called Citric acid.
  • Citric acid (6C) is gradually converted back to the 4-carbon compound ready to start the cycle once more.
  • The carbons removed are released as CO2.
  • The hydrogens, which are removed, join with NAD to form NADH2.

Cytochrome system: hydrogen carrier system; electron transfer system

Cytochrome system - NADH2 molecules produced during glycolysis and the Krebs cycle transfer the hydrogens to the cytochrome system. Hydrogen is carried across with the system of hydrogen carriers. ADP and Pi is converted to ATP. Oxygen accepts hydrogen to produce water.

Most of the energy produced during respiration is made by the cytochrome system. In this stage of aerobic respiration, the NADH2molecules [molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds produced during glycolysis and the Krebs cycle transfer the hydrogens to the cytochrome system.

This is a system of hydrogen carriers located on the cristae of the mitochondrion and this stage also requires oxygen, which acts as the final hydrogen acceptor:

  • The oxygen and the hydrogen combine to form metabolic or respiratory water.
  • If oxygen is not present to act as the final hydrogen acceptor, the hydrogen cannot pass through the system and complete oxidation cannot take place

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